The present invention relates to a gas cycle machine and, particularly, to a generation of cryogenic temperature.
FIG. 5 shows, in partial cross section, a conventional gas cycle machine such as disclosed in Japanese Patent Publication No. 28980/1979. In FIG. 5, a reference numeral 1 depicts a cylinder in which a piston 2 and a free-displacer 3 reciprocate with a phase difference therebetween. A compression space 4 provided between a working surface of the piston 2 and a working surface 3a of the free-displacer 3 holds a cooler 5. An upper working surface 3b of the free-displacer 3 defines a border line of an expansion space 6 which, together with the compression space 4, forms a working space. A regenerator 7 disposed in the free-displacer 3 can communicate through a lower central hole 8 with a working gas existing below and through an upper center hole 9 and a radial duct 10 with a working gas above. A freezer 11 is also included for heat exchange between a cold working gas expanded and a member to be cooled. seals 12 and 13 are provided between the piston 2 and the cylinder 1 and seals 14 and 15 are provided between the free-displacer 3 and the cylinder 1. A sleeve 16 of non-magnetic, light-weight material such as hard paper or aluminum is provided around a lower portion of the piston 2. The sleeve 16 has a movable coil 17 thereon from which lead wires 18 and 19 extend through a wall of a housing 20 connected air-tightly to the cylinder 1, externally. Ends of the lead wires 18 and 19 are connected to electric contacts 21 and 22 disposed externally of the housing 20, respectively. The movable coil 17 can reciprocate axially within an annular gap 23 in which a armature magnetic field is established. Magnetic flux of the magnetic field extends in the gap so that it traverses the moving passage of the movable coil 17. The magnetic field is obtained, in the shown example, by an annular permanent magnet 24 magnetic poles at an upper and a lower end , an annular disc 25 of soft iron, a solid cylinder 26 of soft iron and a circular disc 27 of soft iron. The permanent magnet 24 and the yoke members 25, 26 and 27 form a closed magnet circuit. The piston 2 is provided with a support spring 28 for keeping the center of the piston 2 stably. An upper end of the support spring is locked on a protrusion 29 and a lower end thereof is locked around terminal member 30 to prevent lateral movement thereof. The free-displacer 3 is supported at a lower end thereof by a resilient member 31 by which a stroke of the free-displacer 3 is limited.
In operation, when an alternating current is supplied through the lead wires 18 and 19 connected to the electric contacts 21 and 22 to the moving coil 17, a Lorentz force exerted vertically on the moving coil 17 due to an interaction of the permanent magnetic field in the annular gap 23 and the current flowing therethrough. As a result, an assembly composed of the piston 2, the sleeve 16 and the moving coil 17 starts to vibrate. The vibration of the piston 2 varies the volume of the compression space 4 upon which the working gas filling the working space is compressed and expanded to change gas pressure. This change of gas pressure causes a periodical change of pressure difference across the regenerator 7, resulting in that the free-displacer 3 moves at the same frequency as that of the piston 2 with different phase to each other due to a
time lag between a motion of the free-displacer 3 and
the pressure difference variation.
With the movements of the piston 2 and the freedisplacer 3 in different phases, the working gas such as helium gas in the working space experiences a thermodynamic cycle well known as the Inversed Stirling Cycle resulting in a cold state in the expansion space 6.
Since, in the conventional gas cycle machine, the permanent magnet 24 and the parts 25, 26 and 27 all of which are formed of soft iron constitute a closed magnetic circuit, the size of the circular disc 27 must be large causing the overall size of the machine to be large.